Synthesis, crystal structure and photophysical properties of bis[2,6-difluoro-3-(pyridin-2-yl)pyridine-κN](trifluoromethanesulfonato-κO)silver(I)

The AgI atom in the title compound, which exhibits strong blue emission, adopts a highly distorted trigonal–planar geometry coordinated by two pyridine N atoms of two crystallographically independent 2′,6′-difluoro-2,3′-bipyridine ligands and one O atom of the trifluorometanesulfonate anion.

In the title compound, [Ag(CF 3 SO 3 )(C 10 H 6 F 2 N 2 ) 2 ], the Ag I centre adopts a highly distorted trigonal-planar coordination environment resulting from its coordination by one O atom of the trifluoromethanesulfonate anion and the pyridine N atoms of two crystallographically independent 2 0 ,6 0 -difluoro-2,3 0bipyridine ligands, which display very similar conformations to one another. Pairwise AgÁ Á ÁO-SO 2 CF 3 À [AgÁ Á ÁO = 2.8314 (14) Å ] interactions and intermolecular C-HÁ Á ÁO interactions between inversion-related units lead to the formation of an eight-membered cyclic dimer in which the silver atoms are separated by 6.2152 (3) Å . In the crystal, the dimers are linked through C-HÁ Á ÁO hydrogen bonds, halogenÁ Á Á and weakstacking interactions, resulting in the formation of a three-dimensional supramolecular network. The title compound exhibits a strong and broad emission band from 400 nm to 550 nm in solution and its photoluminescence quantum efficiency is estimated to be ca 0.2, indicating that the title compound could have applications as an emitting material in organic light-emitting diodes (OLEDs).

Chemical context
Recently, great attention has been paid to 2,3 0 -bipyridinebased Ir III and Pt II complexes by many researchers because of their applicability to OLEDs and solid-state lighting (Kang et al., 2021;Reddy & Bejoymohandas, 2016). In particular, 2 0 ,6 0difluoro-2,3 0 -bipyridine complexes of iridium(III) are considered to be strong candidates as both blue triplet emitters in phosphorescent organic light-emitting diodes (PHOLEDs) and single dopants in white organic light-emitting diodes (WOLEDs) (Zaen et al., 2019;Kang et al., 2020;Lee et al., 2018). Despite these investigations, reports regarding the structures and photoluminescence properties of 2,3 0 -bipyridine-based group-11 metal complexes are scarce, and related research is limited (Li et al., 2019). Among the group-11 elements, coordination polymers of Ag I have been demonstrated to exhibit structural diversity as a result of the d 10 configuration of the metal ion . Moreover, the various coordination environments around the Ag I centre are generally constructed by the ligands, solvent molecules, and counter-anions (Lee et al., 2016). Until now, there has been no report with respect to an Ag I complex bearing a 2 0 ,6 0difluoro-2,3 0 -bipyridine ligand as compared to 2,2-bipyridinebased Ag I complexes (Pal et al., 2020). This fact prompted us to investigate the structures and luminescent properties of 2,3 0 -bipyridine-based Ag I complexes: in this study, we report the preparation, structural characterization and luminescent properties of an Ag I complex of 2 0 ,6 0 -difluoro-2,3 0 -bipyridine.

Structural commentary
The asymmetric unit in the title compound consists of an Ag I cation, a CF 3 SO 3 À trifluoromethanesulfonate anion and two crystallographically independent C 10 H 6 F 2 N 2 2 0 ,6 0 -difluoro-2,3 0 -bipyridine ligands, which adopt very similar conformations, such that the dihedral angles between the pyridyl rings in the N1-and N3-containing molecules are 53.11 (5) and 53.10 (7) , respectively. As shown in Fig. 1, the Ag I ion is coordinated by two pyridine N atoms (N2 and N4) from two 2 0 ,6 0 -difluoro-2,3 0 -bipyridine ligands and one O atom from the trifluoromethanesulfonate anion, forming a highly distorted trigonal-planar geometry. Selected bond lengths and angles around the Ag1 atom are given in Table 1: the N-Ag-N and N-Ag-O angles fall in the range 86.55 (5)-148.65 (5) , deviating significantly from an ideal trigonal-planar geometry. This may reflect the influence of an additional AgÁ Á ÁO-SO 2 CF 3 À interaction between the metal ion and an O atom of an adjacent trifluoromethanesulfonate anion [Ag1Á Á ÁO2 i = 2.8314 (14) Å ; black dashed lines in Fig. 2; symmetry code: (i) Àx + 1, Ày, Àz + 1]. The Ag I atom is displaced out of the trigonal N2, N4, O1 coordination plane by 0.1057 (9) Å . The C6-C10/N2 and C16-C20/N4 pyridine rings coordinated to the Ag I centre are tilted by 25.75 (10) with respect to each other. The pairwise AgÁ Á ÁO links lead to the formation of an eightmembered [Ag-O-S-O-] 2 cyclic dimer, in which the silver atoms are separated by 6.2152 (3) Å . The cyclic dimer is consolidated by C-HÁ Á ÁO interactions (Table 2; yellow dashed lines in Fig. 2).

Supramolecular features
In the extended structure, the dimers are linked through C19-H19Á Á ÁO3 hydrogen bonds (Table 2) and weakstacking interactions [yellow and sky-blue dashed lines in Table 1 Selected geometric parameters (Å , ).

Luminescent properties
In CH 2 Cl 2 solution, the title compound exhibits a strong and broad emission band with max = 400 nm, as shown in Fig. 5. This emission band may arise from -* transitions of the bipyridine ligand because the absorption of the title compound is very similar to that of the free ligand. Interestingly, upon the complexation of ligand with the Ag(CF 3 SO 3 ) unit, significant blue-shifted emissions (> 50 nm) are observed as compared with bipyridine based Ir III complexes (Lee et al., 2009). Moreover, a broad emission from 400 nm to 500 nm in the title compound may be due to the predominantly fluorescent emission from the 2 0 ,6 0 -difluoro-2,3 0 -bipyridine ligand because the emission maximum of the free ligand, i.e. phosphorescent emission, occurs at approximately 450 nm (triplet energy, T 1 = 2.82 eV). The observed emission of the title compound is therefore attributed to ligand-centered -* transitions with a minor contribution of an Ag-based metal-to-ligand charge-transfer transition. Similar dual-emission behaviour has been noted for some Ag I complexes with 2-methylthiothiazole (Rogovoy et al., 2019) 1226 The two-dimensional supramolecular network formed through C-HÁ Á ÁO hydrogen bonds (yellow dashed lines), FÁ Á Á (red dashed lines) andstacking (sky-blue dashed lines) interactions. For clarity, H atoms not involved in the intermolecular interactions have been omitted. Atom colours as in Fig. 2.

Figure 5
Absorption and emission spectra of the free ligand and the title compound in solution [concentrations = 1.0 Â 10 À5 M] at room temperature (inset: emission photo); " ' 100,000 M À1 cm À1 . and pyridylphosphine ligands (Baranov et al., 2019). The emission intensity of the title compound was also higher than that of free ligand, as shown in Fig. 5. The photoluminescence quantum efficiency of the title compound was estimated to be ca 0.2 (Fig. 5,inset). Such an efficiency is large enough to potentially use the title compound as the emitting material in an organic light-emitting diode (OLED) application.

Database survey
A survey of SciFinder (SciFinder, 2021) for transition-metal complexes bearing the 2 0 ,6 0 -difluoro-2,3 0 -bipyridine moiety as a ligand gave 25 hits. They include reports about the crystal structures and photophysical properties of Ir III and Pt II complexes based on this ligand (HOVHAC, Lee et al., 2009;OHUMUB01, Lee et al., 2015;JUDZAL, Park et al., 2015). The survey revealed no exact matches for the reported structure of the title complex. To the best of our knowledge, this is the first crystal structure reported for a silver complex with the title ligand.

Synthesis and crystallization
All experiments were performed under a dry N 2 atmosphere using standard Schlenk techniques. All solvents used in this study were freshly distilled over appropriate drying reagents prior to use. All starting materials were purchased commer-cially and used without further purification. The 1 H NMR spectrum was recorded on a JEOL 400 MHz spectrometer. The ligand, 2 0 ,6 0 -difluoro-2,3 0 -bipyridine  was synthesized according to the previous report. The title compound was synthesized as follows: the ligand (0.075 g, 0.39 mmol) in THF (2 ml) was added to Ag(CF 3 SO 3 ) (0.47 g, 1.0 mmol) in MeOH (2 ml) in the dark at room temperature and the mixture was stirred for 10 min. After that, the mixture was slowly evaporated in the air and a dark environment to obtain crystals suitable for X-ray crystallographic analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were positioned geometrically and refined using a riding model: C-H = 0.95 Å with U iso (H) = 1.2U eq (C).

Bis[2,6-difluoro-3-(pyridin-2-yl)pyridine-κN](trifluoromethanesulfonato-κO)silver(I)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.